It is often required of direction-finder antenna systems that they be capable of covering the entire 360 degree azimuth range at an elevation near the horizon with two DF measurement receiver channels. In the past, most devices for achieving this purpose have been limited to a very narrow bandwidth. Consequently, when devices of this type were employed, a large number of them were needed if the frequency band to be monitored was wide.
An antenna whose characteristics are relatively frequency independent throughout a broad bandwidth is the log-periodic antenna. In such an antenna in the prior art, the individual elements are disposed along and perpendicular to an axis. The dimensions of the individual elements are proportional to the distance of the element from a reference point, or vertex, on the axis, and the distances between adjacent elements along the axis are also proportional to the distance from the vertex so that the ratio of the dimensions of one element to those of the previous adjacent element in the array is the same as the ratio for any two other adjacent elements.
Although this log-periodic structure results in a relatively frequency-independent response, radially orienting a number of such structures as sub-arrays of a composite array to achieve a 360 degree azimuth range has not in the past proved satisfactory. The interaction between the individual log-periodic sub-arrays has resulted in direction-finding errors. In addition, these log-periodic antennas have been relatively large which has limited their use on aircraft. Thus, it was previously necessary to employ either a narrow-band device to achieve the 360 degree range, to use extensive azimuth, elevation, and polarization antenna-response calibrations, or to limit the log-periodic structure to a single mechanically/azimuthally steered log-periodic antenna.
Although conventional log-periodic antennas have proven generally suitable for their intended uses, such conventional log-periodic antennas are generally physically too large to be utilized in applications wherein it is desirable that the antenna be as small as possible. Size is particularly important in small aircraft, spacecraft, and missiles. For example, the Unmanned Air Vehicle (UAV) is an unmanned military surveillance aircraft which must fly a considerable distance without refueling. Thus, it is desirable to minimize the weight of the aircraft, so as to increase the effective range thereof. Such weight constraints limit the physical size of any antenna to be utilized. The small size of the UAV also dictates the use of a small antenna and a limited number of RF measurement receivers.
It is known to form such an array of antenna elements in a flat, generally circular configuration so as to define a broadband antenna which requires minimal volume and minimizes the size of the antenna. One example of such a circular, log periodic, broadband antenna is disclosed in U.S. Pat. No. 4,594,595 issued on Jun. 10, 1986 to Keith Struckman and entitled “Circular Log-Periodic Direction—Finder Array”.
Three other examples of circular log periodic antennas are provided in U.S. Pat. No. 4,063,249, issued on Dec. 13, 1997 to Bergander et al., and entitled “Small Broadband Antenna Having Polarization Sensitive Reflector System”; U.S. Pat. No. 5,164,738, issued on Nov. 17, 1992 to Walter et al., and entitled “Wideband Dual-Polarized Multimode Antenna”; and U.S. Pat. No. 5,212,494 issued on May 18, 1993 to Hoffer et al., and entitled “Compact Multi-Polarized Broadband Antenna”.
Another problem with prior art log periodic antennas is as follows. Log periodic slots operate when the operational frequency is approximately λ/2. Therefore, if the log periodic slot array is designed where adjacent parts of the array are touching then the slot of a first part of the array must be spaced at λ/2 from the slot of an adjacent part of the array. This spacing dictates the minimum size of the DF antenna.
Thus, there is a need in the art to provide a broadband, log-periodic antenna assembly which is comparatively small in size and therefore does not contribute substantially to the weight of an airborne vehicle to which it is attached and is also suitable for use in very small vehicles. Further, there is a need for a log periodic antenna that is broadband, and that can achieve equal antenna gain in all directions at or near the horizon.